It is known, for example from Nakazawa et al. U.S. Pat. No. 4,542,356 issued Sep. 17, 1985 and entitled "High Frequency Narrow-Band Multi-Mode Filter" and from M. Tanaka et al., "Narrow Bandpass Filter Using Double-Mode SAW Resonators On Quartz", 38th Annual Frequency Control Symposium, pages 286-293, 1984, to provide a double/node surface wave resonator in which two inter-digital transducers (IDTs) are arranged side by side in close proximity to one another on a piezoelectric substrate to provide surface wave coupling between them, each IDT also preferably being positioned longitudinally between two grounded reflector gratings via which a ground connection is made to a common bus bar or rail of the two IDTs. The outer rail of one IDT provides an input, relative to ground, of the resonator and the outer rail of the other IDT provides an output, relative to ground, of the resonator.
Such a resonator is useful as a filter at high frequencies, for example as an IF (intermediate frequency) filter in cellular radio equipment. However, the resonator provides a single-ended or unbalanced input and output, whereas in general it is preferred that radio circuits handle differential or balanced signals in order to provide better linearity and greater bandwidth. In order to use the known resonator in such balanced circuits, baluns (balanced-to-unbalanced signal transformers) are required. However, the use of baluns in large-scale manufacture is undesirable in view of issues of reliability and electromagnetic radiation. Furthermore, the relatively large size of baluns excludes them from applications using multi-chip modules. Small size is an important factor in cellular radio equipment.
It is also known, from P. S. Cross et al., "Synchronous IDT SAW Resonators With Q Above 10,000", 1979 Ultrasonics Symposium, pages 824-829, 1979, to provide a SAW resonator in which the IDT electrodes are positioned in spatial synchronism with the fingers of reflection gratings to produce a short resonant cavity which inherently suppresses spurious longitudinal modes.
An object of this invention is to provide an improved surface wave resonator, which can be used in a balanced circuit without requiring baluns.